A magnetic inspection apparatus detects a defect, e.g., a flaw or an inclusion, in the interior or on the surface of a thin steel strip by utilizing magnetism. A magnetic inspection apparatus for a thin steel strip which can not only examine a thin steel strip as an object to be examined in a still state but which also can continuously detect defects present in a thin steel strip traveling along, e.g., a manufacture, line of a factory and the like, has been disclosed in Published Unexamined Japanese Utility Model Application No. 63-107849.
FIGS. 31 and 32 are sectional views taken along different directions, respectively, of an above-described magnetic inspection apparatus for a thin steel strip which continuously detects defects of a traveling thin steel strip.
A hollow roller 1 is made of a non-magnetic material. One end of a stationary shaft 2 extends along the axis of the hollow roller 1. The other end of the stationary shaft 2 is fixed on the frame of a base (not shown). The stationary shaft 2 is supported on the inner circumferential surfaces of the two ends of the hollow roller 1 by a pair of rolling bearings 3a and 3b so that it is located along the axis of the hollow roller 1. Hence, the hollow roller 1 freely rotates about the stationary shaft 2 as the central axis of rotation.
Inside the hollow roller 1, a magnetization core 4c having a substantially U-shaped section is fixed on the stationary shaft 2 through a support member 5 such that the magnetization core 4c's magnetic poles 4a and 4b, which constitute a magnetic path, are close to the inner circumferential surface of the hollow roller 1. A magnetization coil 6 is wound around the magnetization core 4c. Hence, the magnetization core 4c with the magnetic poles 4a and 4b, and the magnetization coil 6 constitute a magnetizer 4. A plurality of magnetic sensors 7 are arranged between the magnetic poles 4a and 4b of the magnetization core 4c of the magnetizer 4 in the axial direction. Each magnetic sensor 7 is fixed on the stationary shaft 2.
A power cable 8 for supplying an excitation current to the magnetization coil 6 and a signal cable 9 for deriving the respective detection signals output from the respective magnetic sensors 7 extend to the outside through the interior of the stationary shaft 2. Hence, the positions of the magnetization core 4 and the respective magnetic sensors 7 are fixed, and the hollow roller 1 rotates at a small gap to the outer surfaces of the magnetizer 4 and the respective magnetic sensors 7.
When the outer circumferential surface of the hollow roller 1 of the magnetic inspection apparatus having such an arrangement is urged against one surface of a thin steel strip 10, traveling in the direction of an arrow A, at a predetermined pressure, the hollow roller rotates in the direction of an arrow B as the stationary shaft 2 is fixed on the frame of the base.
When an excitation current is supplied to the magnetization coil 6, a closed magnetic path is constituted by the magnetization core 4c and the traveling thin steel strip 10. Therefore, if a defect described above is present in the interior or on the surface of the thin steel strip 10, the magnetic path within the thin steel strip 10 is disturbed, and magnetic flux leakage occurs. The magnetic flux leakage is sensed by a magnetic sensor 7 at a corresponding position and is detected as a defect signal.
The signal level of the detected defect signal corresponds to the size of the defect in the interior or on the surface of the thin steel strip 10. Therefore, the presence and size of the defect of the thin steel strip 10 can be obtained in the form of the signal level of the defect signal.
However, the signal level of a defect signal is largely changed depending on the state of the magnetic path constituted by the thin steel strip 10 and the magnetizer 4 comprising the magnetization core 4c and the magnetization coil 6, a distance L between the magnetizer 4 and the thin steel strip 10, a distance l between the thin steel strip 10 and the respective magnetic sensors 7, which is called a lift-off distance, and so on.
In order to eliminate these drawbacks, the distance L between the thin steel strip 10 and the magnetizer 4 and the distance l between the thin steel strip 10 and the respective sensors 7 are constantly kept to be predetermined values by using the hollow roller 1 having a predetermined thickness t, as shown in FIGS. 31 and 32. If the hollow roller 1 is made of a magnetic material, formation of the magnetic path into the thin steel strip 10 is interfered. Therefore, the hollow roller 1 is made of a non-magnetic material.
Accordingly, the smaller the thickness t of the hollow steel strip 1, the smaller the distance L between the magnetic poles 4a and 4b of the magnetizer 4 and the thin steel strip 10, and the larger the magnetic field formed within the thin steel strip 10, thereby obtaining stable magnetic fluxes. For this reason, it is preferable that the thickness t of the hollow roller 1 be made small.
If the hollow roller 1 has a large thickness t, its moment of inertia becomes large. Then, when the travel speed of the thin steel strip 10 fluctuates, a sliding phenomenon may occur between the contact surfaces of the hollow roller 1 and the thin steel strip 10, which sliding phenomenon may damage the surface of the thin steel strip 10. Therefore, the 1 moment of inertia must be decreased by decreasing the thickness t of the hollow roller 1. When only the moment of inertia is to be decreased, the outer diameter of the hollow roller 1 may be set small. The outer diameter is restricted by the size of the magnetizer 4 or the magnetic sensors 7 housed in the hollow roller 1.
As described above, in order to continuously detect a defect in the traveling thin steel strip 10 with a high precision, as described above, the surface of the thin steel strip 10 must be in constant contact with the outer circumferential surface of the hollow roller 1. As a result, a downward force caused by the tension of the thin steel strip 10 and a downward force caused by the weight of the thin steel strip 10 itself are applied to the hollow roller 1. When a downward force is applied, the hollow roller 1 is deformed or damaged. Then, the distance L between the thin steel strip 10 and the magnetizer 4 and the distance l between the thin steel strip 10 and the respective magnetic sensors 7, described above, cannot be controlled to be the predetermined values. As a result, a defect detection precision may be degraded, or inspection may become impossible.
Hence, if the true circle state of the hollow roller 1 is to be kept over a long period of time, the thickness t of the hollow roller 1 should not be made smaller than a predetermined limit. For example, under the condition that the travel speed of the thin steel strip 10 is 100 m/min., the limit thickness t is about 2 mm.
The strength of the magnetic field generated by the magnetizer 4, housed in the hollow roller 1, comprising the magnetization core 4c and the magnetization coil 6 may be increased. However, if the size of the magnetization core 4c or the intensity of the current supplied to the magnetization coil 6 is increased over a predetermined limit, the entire apparatus must be made large, with the manufacturing costs being greatly increased.